The present invention relates to a contour compensator for adding preshoot or overshoot to a video signal that includes luminance and chromaticity components. The preshoot or overshoot is added while the video signal is in transition between two levels. More particularly, the invention relates to a circuit for interrupting or controlling the amount of preshoot or overshoot to be added to a transitional portion of the video signal in which supplemental information (such as a news flash, weather bulletin, or a movie subtitle) overlaps the picture.
Resolution of a picture, that corresponds to a video signal reproduced by a tuner of a television receiver or reproduced by a magnetic record/playback apparatus, is enhanced by adding preshoot or overshoot to the video signal when its amplitude is in transition. This addition is called contour compensation and relates to the slope of the video signal's amplitude. For instance, preshoot or overshoot is added to the transitional portion of a luminance signal, such as when the signal changes from a black to a white level to emphasize the contrast between white and black and enhance the picture resolution.
However, depending on the picture's content, such contour compensation may deteriorate picture quality by adding a constant amount of contour compensation to the amplitude of a video signal, that is in transition, regardless of the content of the picture. More specifically, a constant amount of compensation may be added, regardless of the amount of correlation between scan lines of a video signal or the magnitude of a particular frequency component of the video signal. Thus, the amount of contour compensation will be excessive for a picture showing swift movement or including many details and the resulting picture will be rough. Specifically, as the intricacy of a picture increases the magnitude of the high-frequency component also increases. Adding a large amount of preshoot or overshoot to the video signal for this detailed picture causes the picture to deteriorate since the preshoot or overshoot dominates the video signal.
To solve this problem, an improved contour compensator has been proposed which compensates for contour based on the amount of correlation between video scan lines and the magnitude of the high frequency component of the video signal in order to control the amount of contour compensation. For instance, the improved contour compensator increases the amount of contour compensation for a portion of a video signal that has a high degree of correlation, such as in a video portion having slight movement. The contour compensation is also increased in a video portion having a small high-frequency component, such as in a coarse picture having few details. Similarly, the improved contour compensation reduces the amount of compensation for portions of the video signal having a small degree of correlation between scanning lines or for video signal portions having a large high-frequency component, thereby performing proper contour compensation according to the contents of the picture.
Meanwhile, in the video signal, there are times when character information overlaps the periphery of a picture. For instance, when a news flash or weather bulletin overlaps the video signal transmitted from a television station. This additional independent information is usually presented at the bottom of the picture. Further, captions from movies, such as subtitles may overlap the video signal recorded on a magnetic recording apparatus. Video signals having this overlapping information are different from the original signals, and thus experience radical level transitions. If the contour compensation signal is added based upon these radical transitions, which represent an incorrect video signal, the picture becomes unnatural. This unnatural characteristic results from the fact that a video signal containing overlapping information has a different S/N ratio than a video signal without overlapping information. Thus, when a video signal has overlapped information on the picture's periphery, the contour compensation signal or the amount of contour compensation added to the periphery of the picture must be interrupted or controlled.